Resin-based composite and restorative materials generally have very high cohesive strength, and accordingly are widely used in dentistry. However, in recent years there has been a resurgence in water-based cements. These water-based cements may contain resinous components, but are distinguished by containing substantial amounts of water. Examples include metal oxide cements such as those described in U.S. Pat. No. 3,655,605 and fluoroaluminosilicate glass cements (also known as "glass ionomer cements") such as those described in Example 6 of the '605 patent and in U.S. Pat. Nos. 3,814,717, 4,043,327, 4,143,018 , and 4,209,434. These water-based cements have also found utility in medical applications such as the fabrication of orthopedic bandages, as described in U. S. Pat. Nos. 4,043,327 and 4,243,567.
Typically, these cements are cured or hardened by combining a polyfunctional acid, water and an acid-reactive metal oxide or glass filler. Hardening occurs due to the reaction between the acidic groups of the polyfunctional acid and cations leached from the filler. More recently, light-curable water-based cements have appeared. Examples are shown in U.S. Pat. No. 4,872,936, European Pat. Application Nos. 0 323 120 and 0 329 268 and Australian Published Pat. Specification No. 46717/89. These light-curable cements include one or more ethylenically-unsaturated components and a suitable photoinitiator. Hardening takes place independently from the acid-filler reaction mentioned above via crosslinking of the ethylenically-unsaturated component upon exposure of the photoinitiator to light or other activating energy.
A light-curable and apparently anhydrous cement is shown in European Pat. Application No. 0 391 619. It contains a number of ingredients, including benzoyl peroxide.
Although light-curable cements have many advantages, they do require use of a light. If the light is defective (for example, due to breakage or discoloration of the filters typically installed in the light path or through deterioration of the curing lamp), then the composition may undergo incomplete light-curing polymerization. Also, owing to the need to insure adequate penetration of the light energy into the cement, thick restorations typically must be built up in separately cured thin layers. In some dental applications it may be impractical to use a curing light. For example, when luting a metallic crown to a prepared tooth stump, light typically will not penetrate underneath the crown. Likewise, for endodontic applications, light may not penetrate the full depth of the endodontic preparation. On the other hand a cement that cures only by dark reactions takes a long time to cure. During this time the cement may be prone to moisture contamination.
Accordingly, presently available cements have not had universal applicability. This has led to the appearance of many special-purpose products in the marketplace, with attendant need for the practitioner to maintain separate inventories of each cement, to undergo training in the use of more than one cement, and to avoid improper use through inadvertent selection of the wrong cement.